1. Field of the Invention
The invention relates to wireless communication systems and, in particular, to an advanced method and system for providing RAKE delay control.
2. Description of the Related Art
In wireless communication, the physical channel between a transmitter and a receiver is formed by a radio link. In most cases, the transmitter is not narrowly focused towards the receiver and, in addition to a possible direct path, many other propagation paths exist between the parties due to objects in the surroundings. Referring to FIG. 1, for example, a receiver 100 may receive a radio signal from a transmitter 102 on a direct and unobstructed path (Path 1). However, many other propagation paths (e.g., Path 2, Path 3) may exist. Thus, multiple instances of the same transmission may be received by the receiver 100 as portions of the radio signal are reflected by various objects and obstacles (e.g., a house 104, a building 106) in the surroundings.
These multipath signals will arrive at the receiver 100 delayed by different amounts of time after the direct signal and will usually have different magnitudes than the direct signal. Multipath signals with similar propagation distances may then be combined, depending on the time resolution of the transmission system and the instantaneous phase relationship of the multipath signals, to form a distinct multipath component. The effect of combining depends on the instantaneous relationship of the carrier wavelength and distance differences and, in the case of destructive interference, can lead to significant decrease of the magnitude, or fading, of the path gain.
In CDMA (Code Division Multiple Access) based systems, a RAKE receiver is used to identify and track the various multipath components for a given channel. The RAKE receiver includes a plurality of despreaders or RAKE fingers, each of which is assigned a multipath component. The despreaders each have a copy of the CDMA spreading code that is delayed by an amount of time equal to the path delay of the corresponding multipath component. The outputs of the despreaders are then coherently combined to produce a symbol estimate.
In order to be effective, the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for every path. The reason is because paths that are not detected can still act as sources of interference to the other fingers in the RAKE, even though the signal energy they carry is not usefully utilized. In addition, the smaller the number of paths available at the receiver (utilized diversity), the larger the probability they may undergo simultaneous deep fade, leading to serious degradation of the block error rate (BLER).
Techniques for identifying a multipath signal are described, for example, in U.S. patent application Ser. Nos. 09/678,165, 10/246,873 and 10/246,874, which are hereby incorporated by reference herein. One way to identify a multipath signal and determine its delay is to search for possible paths over a range of possible delays. This path searching can be performed by transmitting a pilot signal from the transmitter and applying a series of predefined delays for despreading at the receiver. Where the predefined delays happen to coincide with the arrival times of the multipath signals, a larger-magnitude channel estimate will result. The resulting delay profile, which can be a complex delay profile (CDP) or a power delay profile (PDP), may then be subjected to peak detection, and the location of the peaks are reported to the RAKE receiver as estimates of the multipath delays of the channel.
FIG. 2 illustrates an exemplary PDP of a given channel for one pass or iteration of the path search. The vertical axis in FIG. 2 represents the magnitude of the detected signal, while the horizontal axis represents the size of the delays applied. The PDP of FIG. 2 shows all the signals that are received by the receiver, including noise and interference signals. Only the peaks in the PDP correspond to the multipath signals of the channel. The peaks together form the impulse response of the channel.
However, the processing and power consumption expense of frequently executing this path searching routine is prohibitive in many cases. Therefore, it is necessary to introduce compromises that make the solution feasible. Thus, a practical implementation may use reduced searcher resolution, and may introduce additional, short-range despreader groups to produce higher-resolution estimates of certain areas of the PDP. An example of this type of architecture can be seen in published PCT application WO0035112 and in FIG. 3.
Referring to FIG. 3, an exemplary implementation of a RAKE delay controller (RDC) 300 includes the use of a path searcher 302, a path tuning stage 304, and a controller 306, all interconnected as shown. The path searcher 302 is a device that computes instantaneous channel impulse response estimates (complex or power) over a range of delays that constitutes a significant fraction of the maximum delay spread allowed by the system. The CDP or PDP for a given delay value is estimated by correlating the received data for the pilot symbols with an appropriately delayed copy of the spreading sequence, a method which is well known in the art. Often, the path searcher 302 is used mainly as a means to detect the existence of paths and, therefore, its output resolution may be somewhat lower than the resolution required by the RAKE receiver.
The path tuning stage 304 produces a high-resolution instantaneous CDP or PDP over a narrow delay window. The path tuning stage 304 has tuning fingers that may include a number of despreaders that are similar to the despreaders of the RAKE fingers of the path searcher except they are usually more closely spaced together. Because of the higher resolution, the path tuning stage 304 is commonly used to locally refine the coarser PDP information provided by the path searchers 302.
The controller 306 extracts the physical path location information from the path searcher 302 and the path tuning stage 304 output. This location information is then presented as delay estimates to subsequent receiver stages, and assignment of distinct paths to the RAKE fingers is made. The degree of complexity of the controller may vary significantly depending on system parameters, and may range from simple peak detection to sophisticated de-convolution and filtering algorithms.
The nature of the RDC 300 shown in FIG. 3 is that it is an inherently sequential structure insofar as only the most recently produced path searcher results are used when computing the refined delay estimates. Also, when new path searcher output estimates become available, the locations used for fine-tuning by the tuning fingers are reassigned according to the new estimates. Thus, the fine-tuning process does not explicitly follow or account for the existing path positions. That is, inclusion of a path in RAKE processing depends only on the path's fading state. Due to the fading of the individual paths and varying levels of interference, some multipath components may escape detection altogether, thereby degrading both the instantaneous signal-to-interference ratio (SIR) and the utilized diversity in the fading environment.
Accordingly, it would be desirable to provide a RAKE delay control architecture that is capable of tracking the presently known paths over time and merging the tracking results with the new path searcher results. Once the paths are assigned, it would be desirable for the assignments to remain constant over a significant time to allow reliable power and interference estimation. It is further desirable that such a RAKE delay control architecture can be used in devices where the resources (e.g., computational load, power) are limited. Additional support functions, such as the ability to determine the path searcher search area and activation times, and placement of the tuning fingers, are also desirable.